This invention relates to the field of cardiopulmonary resuscitation. In particular, the invention provides improved devices and methods for enhancing blood circulation in patients undergoing cardiopulmonary resuscitation (hereon abbreviated as CPR). Such procedure is applied, for example, when cardiac arrest is present. In these situations, the heart ceases to pump blood out of the heart. To obtain some circulation until the normal pumping action of the heart can be restored, manual compressions are conventionally applied on the chest of the supine patient. The compressions on the chest may be alternated with brief periods of forced breathing into the patient, for example, by mouth to mouth ventilation. Alternatively, a ventilation bag with facemask or tracheal tube may be used to achieve the same effect. The American Heart Association publishes guidelines on CPR procedures. For example, the “2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care”, published in the Circulation journal, give a good overview of the subject of CPR.
While manual compressions are partially effective in providing circulation to the patient, it is not a perfect method. The manual compressions applied on the chest attempt to squeeze the heart and major vascular structures to eject blood into the extrathoracic arterial circulation. However, the rib cage provides an obstacle to achieve effective squeezing of the heart and vascular structures. The rib cage, in fact, spatially protects the internal organs including the heart from external forces. As a result, the physical frame forming the rib cage attenuates the amount of squeezing on the heart obtained by external compressions on the chest, by distributing the force across entire chest and rib cage.
Furthermore, when a rescuer provides CPR and compresses the chest of the patient, the heart only experiences a partial squeeze, because soft tissues surround the heart and mediastinum. Namely, the soft tissues are the lungs on the sides of the mediastinum, and inferiorly, the soft tissues of the upper abdomen. As the external compression is delivered, the heart deforms and expands part of its volume into the surrounding soft tissues. This expansion creates inefficiencies in squeezing the heart during CPR. It would be desirable to impede that lateral expansion into soft tissues so that a more effective cardiac squeeze is achieved. One such method to effectively accomplish such lateral support is open chest cardiac massage, in which clinicians manually squeeze the heart with their hands. In this case the squeeze of the heart is delivered around most of the heart's perimeter, not just the front and back as in traditional CPR. The squeeze is therefore very effective, but it of course requires a very invasive surgery to expose the heart, and is thus not amenable to typical CPR and first aid situations. In any case, the point emphasized here is the inefficiency of the squeeze of the heart due to its laterally surrounding soft tissues and its protective rib cage, as provided by conventional CPR methods.
In an effort to alleviate some of the above shortcomings, and to enhance circulation during CPR, several devices have been proposed in prior art. For example, U.S. Pat. No. 5,551,420 to Lurie describes a special valve coupled to the airway of the patient, such that the flow of air into the patient's lungs is restricted during the chest decompression phase of CPR. The valve's restriction of air inflow into the patient's lungs, in combination with the natural elastic recoil of the chest after a compression, causes a negative intrathoracic pressure. This vacuum helps draw venous blood from the body into the thorax prior to the next chest compression, thereby better priming the heart pump with enhanced filling. As a result, more blood is in the heart when the next compression occurs, and therefore, more blood is ejected, obtaining enhanced circulation.
In the above cited '420 patent, Lurie also mentions the use of positive pressure, by implementing a restriction to outflow of air from the patient's lungs during the compression phase of CPR. It can be appreciated that if the airway is restricted to outflow, greater intrathoracic pressure will be obtained during a compression step of CPR. Such enhanced pressure will help develop a more efficient ejection of blood from the heart. This addresses the inefficiency of cardiac expansion of the heart into surrounding soft tissues during external compression. Because the lungs cannot readily evacuate their air due to the outflow restriction, the heart is laterally impeded from expanding into the lung spaces. This contributes to a more effective squeeze of the heart when applying external compression to the front of the chest.
The prior art however does not describe a sequence, nor a device to provide it, that would combine optimized positive and negative pressures. Furthermore, when passive decompression CPR is used according to the known art, there is a disadvantage when providing inflow air-resistance during more than a few compression cycles. The distinction of active and passive decompression in CPR merits explanation at this point. By passive decompression CPR it is understood that no active devices are used to expand the chest after each compression step, for example, by using suction cups on the skin to pull and expand the chest. In passive decompression CPR, the chest is allowed to naturally and elastically recover in shape after each compression. The discussion below, and for the rest of this document, is framed in the context of passive decompression CPR, which is the most commonly used method.
Describing the disadvantage in more detail, when using the known inflow restriction devices, there will be less air exchange occurring than there would be if no air restriction was present. In consequence, there will be less air volume present in the lungs just prior to the compression phase of CPR. In other words, after a few compression-decompression cycles, the patient's chest will hold less air volume at the end of the chest decompression phase, due to the impediment presented by the special valve, which restricts the filling of the lungs. Air is easily ejected from the lungs with chest compression and an open airway, but not so easily inhaled through the restrictive valve. Therefore, the chest will not inflate fully to its natural relaxed state. This volume deficiency will be greater if the cracking pressure is set to a higher value on the inflow restriction valve. The cracking pressure is the pressure at which the valve will open to allow air inflow to the lungs, when the valve is subjected to negative pressure at the patient airway side. It can also be understood as the amount of inflow resistance. It must be properly set for the particular patient, as a child, for instance, may have different negative pressure requirements than a large adult.
The extreme situation of lung air volume reduction occurs with a very high cracking pressure: the air inflow is completely occluded when the chest attempts to expand during the decompression phase of CPR, and no new air enters the chest. Notice that this happens even the though the elastic recoil of the chest creates a relatively high vacuum to draw blood to the heart from the periphery. So while blood is adequately drawn into the chest by vacuum, it is done at the expense of air intake.
The disadvantage noted above has two implications: first, barring manually delivered ventilations, which defeat the negative pressure advantages, there is less respiratory gas exchange with the outside atmosphere than in traditional open airway CPR, so oxygen and carbon dioxide transport is negatively affected. Second, if a device or method were to simply combine vacuum with a positive pressure technique as described earlier (restricting air outflow during chest compression to enhance ventricular blood ejection), it will be less effective. This inefficiency of the compression phase of any such simple combination has not been noted in the prior art. The inefficiency occurs because, with the reduced volume of lung air present at the beginning of the chest compression, the heart and major vessels can more easily deform and expand into the less inflated lung space. In contrast, if the precise states of the lungs and heart were taken into account, for example, if the lungs were instead optimally full of air, and the outflow of air restricted during chest compression, the squeeze on the heart would be enhanced, as inflated lungs present a better lateral obstruction to the heart, than do deflated lungs. Such is one of the objectives of the invention. Similarly, if a vacuum were to be applied without regard to the prior states of the cardio-pulmonary system, the benefit of the negative pressure may not be optimal. Therefore, an optimized combination of vacuum and positive pressures is sought in order to further enhance cardio pulmonary circulation. Further, it would be desirable to accomplish such combination without significantly impairing ventilation of the patient. What is also needed is a device and method that optimally provides both negative and positive intrathoracic pressures to enhance circulation during CPR, but does so while maintaining a degree of gas exchange that does not substantially defeat the assistive thoracic pressures.
The invention embodiments described in this document address these needed characteristics, while offering further advantages, and will therefore provide for enhanced CPR devices and methods.